[FIXED] How to compare members of a list of complex objects with Hamcrest?

Issue

Let's say I have a List<A> where

class A {
    private Integer val;
    private String name;
}

and in my test case I get this list, with uncertain size, and content. What I wish to do is to compare val field of two list elements that I know has to be in the list with given name fields;

List<A> list = logic.getList();
assertThat(list, allOf(hasItems(hasProperty("name", equalTo("first")), 
                       hasItems(hasProperty("val", equalTo(***value from another member with name = "second"))));

How can I achieve this, or is this even possible with Hamcrest Matchers?

Solution

You can implement custom Matcher for your needs, e.g. to check that some items with names has same values fields:

final class FooTest {

    static final class Foo {

        final int val;
        final String name;

        // all args constructor
    }

    // custom matcher
    static final class FoosHasSameValues extends TypeSafeMatcher<List<Foo>> {

        private final Set<String> names;

        // all args constructor

        FoosHasSameValues(final String... names) {
            this(new HashSet<>(Arrays.asList(names)));
        }

        @Override
        protected boolean matchesSafely(final List<Foo> items) {
            final List<Integer> values = items.stream()
                // filter only items with specified names
                .filter(i -> this.names.contains(i.name))
                // select only values
                .map(i -> i.val)
                .collect(Collectors.toList());
            if (values.size() != this.names.size()) {
                // matching failed if list doesn't contains all
                // needed items with names
                return false;
            }
            // check https://stackoverflow.com/a/29288616/1723695
            return values.stream().distinct().limit(2).count() <= 1;
        }

        @Override
        public void describeTo(final Description description) {
            description.appendText("has items [")
                .appendValue(String.join(", ", this.names))
                .appendText("] with same values");
        }
    }

    @Test
    void testMatchers() throws Exception {
        MatcherAssert.assertThat(
            Arrays.asList(
                new Foo("first", 1),
                new Foo("second", 1),
                new Foo("third", 2)
            ),
            new FoosHasSameValues("first", "second")
        );
    }
}

Answered By - Kirill
Answer Checked By - Pedro (JavaFixing Volunteer)